Hi!
After installing OpenDyslexic, open the app and tap the button to install the fonts.
Try using it in an application with a font selection after (pages, numbers are good to test with because they do not hide the font choices). If it does not appear, you may need to ensure the app is closed before opening it again.
Not every app uses Apple’s method for checking available fonts. Some still have it hard-coded from years ago. The ones that use the proper methods will show OpenDyslexic in the font menus after install.